Comparator

ABSTRACT

A comparator comprises transistors to turn on/off an input terminal and a control circuit to enable the transistors in a high impedance input state. The high impedance input state can be detected by the above construction without additionally providing any wire and connector.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a comparator.

2. Related Background Art

FIG. 3 shows an example of a circuit of a conventional comparator. V₃ denotes a reference voltage source; I₇ indicates a constant current source; Q₂₁ to Q₂₅ transistors; and R₁ and R₂ resistors.

A circuit which is constructed by the constant current source I₇ and the transistors Q₂₁ to Q₂₄ is a simple comparator. An input terminal IN and the resistor R₂ are connected to a base of the transistor Q₂₁ and a base of the transistor Q₂₅ through wires and connectors from another electronic circuit board, respectively.

The operation of the comparator of FIG. 3 will now be described hereinbelow.

In the case where the input terminal IN and the resistor R₂ are respectively connected to the above transistors Q₂₁ and Q₂₅, the transistor Q₂₅ is turned off and the circuit operates as an ordinary comparator.

When the terminal IN and the resistor R₂ are not connected, an output of the comparator is an output in a state in which a high level signal has been applied to an input. In this state, if a problem logically occurs, the transistor Q₂₅ connected by the resistor R₁ is set into a saturation state by a power source V_(cc), thereby fixing a base potential of the transistor Q₂₁ to a low level and satisfying the logic.

The conventional comparator, however, has the following technical problems to be solved.

(1) Since the wires or connectors must be added to know the coupling state, the costs rise.

(2) Since the wires or connectors are needed to know the coupling state in addition to a purpose of the input of the comparator, it is impossible to know the coupling states of both of them. Then a disconnection to each wire or a damage of the connector occurs, the normal operation cannot be performed and there is a fear of malfunction.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a comparator which can solve the foregoing problems.

Another object of the invention is to provide a comparator comprising transistors to control the state of an input terminal and a control circuit to enable the transistors in a high impedance input state.

According to the invention, the high impedance input state can be detected by the above construction without additionally providing any wire and connector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a comparator according to an embodiment of the invention;

FIG. 2 is a circuit diagram of a comparator according to another embodiment of the invention; and

FIG. 3 is a circuit diagram of a conventional comparator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a circuit constructional diagram showing most preferably a feature of the invention. In FIG. 1, V₁ denotes a reference voltage source; I₁ to I₃ indicate constant current sources; and Q₁ to Q₁₀ indicate transistors.

A circuit which is constructed by the constant current source I₂ and the transistors Q₄ to Q₇ is a comparator. Current mirror circuits are constructed by the transistors Q₂ and Q₃ and the transistors Q₈ and Q₉. The constant current source I₁ and the transistor Q₁ supply a constant current to the current mirror circuit. The constant current source I₃ and the transistor Q₁₀ supply a constant current to the current mirror circuit.

It is now assumed that the transistors Q₁, Q₄. Q₅, and Q₁₀ are equal PNP transistors and that the transistors Q₂, Q₃, Q₈, and Q₉ are equal NPN transistors. There are the relations of I₁ =I₃ and I₁ >I₂ among the constant current sources I₁ to I₃.

The operation of the comparator of FIG. 1 will now be described hereinbelow.

In the case where some input exists at the input terminal IN which is connected to a base of the transistor Q₄, a collector of the transistor Q₃ current of the transistor Q₁ from the base of the transistor Q₄ and the terminal IN. At this time, the circuit operates as an ordinary comparator.

The operation in a disconnected state of the input terminal IN, which is a problem in the conventional comparator, in the case of the embodiment will now be described. When no input exists at the input terminal IN, the collector of the transistor Q₃ tries to pull in all of the currents which are equivalent to the base current of the transistor Q₁ from the base of the transistor Q₄. Since the relation between the constant current sources I₁ and I₂ is now set to I₁ >I₂, the, base current of the transistor Q₁ should equal the collector current of the transistor Q₃ according to the well known operation of current mirror circuitry. But the collector current of the transistor Q₃ cannot be greater than the base current of the transistor Q₄. Since transistor Q₃ cannot pull in sufficient current from base of Q₄ to equal the base current of transistor Q₁, the transistor Q₃ is set into the saturating state. A base potential of the transistor Q₄ , therefore, is determined by a voltage between the collector and the emitter of the transistor Q₃ and is fixed to a low level.

FIG. 2 is a circuit constructional diagram showing another embodiment of the invention. In FIG. 2, V₂ denotes a reference voltage source; I₄ to I₆ constant current sources; and Q₁₁ to Q₂₀ transistors.

A circuit which is constructed by the constant current source I₅ and the transistors Q₁₄ to Q₁₇ is a comparator. Current mirror circuits are constructed by the transistors Q₁₂ and Q₁₃ and the transistors Q₁₈ and Q₁₉. The constant current source I₄ and the transistor Q₁₁ function as a constant current source which is supplied to the current mirror circuit. The constant current source I₆ and the transistor Q₂₀ function as a constant current source which is supplied to the current mirror circuit.

It is now assumed that the transistors Q₁₁, Q₁₄, Q₁₅, and Q₂₀ are equal NPN transistors and that the transistors Q₁₂, Q₁₃, Q₁₈, and Q₁₉ are equal PNP transistors. There are the relations of I₄ =I₆ and I₄ >I₅ among the constant current sources I₄ to I₆.

The operation of the comparator of FIG. 2 will now be described hereinbelow.

In the case where some input exists at the input terminal IN which is connected to a base of the transistor Q₁₄, a collector of the transistor Q₁₃ supplies a current which is equivalent to a base current of the transistor Q₁₁ to the base of the transistor Q₁₄ and the terminal IN. At this time, the circuit operates as an ordinary comparator.

In the case where no input exists at the input terminal IN, which becomes a problem in the conventional comparator, the collector of the transistor Q₁₃ tries to supply all of the currents equivalent of the base current of the transistor Q₁₁ to the base of the transistor Q₁₄ . Since the relation between the constant current sources I₄ and I₅ is set to I₄ >I₅, the base current of the transistor Q₁₁ should equal the collector current of the transistor Q₁₃ according to the well known operation of current mirror circuitry. But the collector current of the transistor Q₁₃ cannot be greater than the base current of the transistor Q₁₄. Since transistor Q₁₃ cannot supply sufficient current to the base of Q₁₄ to equal the base current of transistor Q₁₁, the transistor Q₁₃ is set into the saturating state. Therefore, the base potential of the transistor Q₁₄ is determined by the voltage between the collector and the emitter of the transistor Q₁₃ and is fixed to a high level.

As described above, according to the embodiments, since none of the wires and connectors is additionally needed, the input disconnection state can be certainly known without increasing the costs. 

What is claimed is:
 1. A comparator comprising:a pair of transistors (Q₄, Q₅) connected to an input terminal (IN) to which an input signal is supplied and to an output terminal (OUT) for supplying an output signal, a base of one of the pair of transistors being connected to the input terminal, and a collector of the other of the pair of transistors being connected to the output terminal; a control circuit (Q₁, Q₂, Q₃, I₁) connected to the input terminal for controlling a potential of the input terminal responsive to a high impedance condition of the input terminal; and a circuit (Q₈, Q₉, Q₁₀, I₃) being identical with said control circuit, and being connected to one (Q₅) of the pair of the transistors connected to said output terminal.
 2. A comparator according to claim 1, wherein said control circuit includes a first transistor (Q₃) connected to the input terminal, a second transistor (Q₂) constituting a current mirror circuit together with the first transistor (Q₃), and a current source (I₁) and a third transistor (Q₁) for supplying a current to the current mirror circuit. 